A bomb explosion in northwestern Syria has damaged part of a wall of the Citadel of Aleppo in the country’s historic Old City.

The Syrian Arab News Agency, SANA, said the blast was carried out by Takfiri terrorists near the 13th century citadel, which is on the list of the United Nations Educational, Scientific and Cultural Organization’s World Heritage sites.

The Britain-based so-called Syrian Observatory for Human Rights also said the explosion occurred in a tunnel near the area.

"A powerful explosion was heard after midnight Saturday. It was caused by the blowing up of a tunnel in the Old City near the Aleppo Citadel,” the observatory’s chief, Rami Abdel Rahman, said in a statement.

There has been no claim of responsibility for the bombing. The ISIL terrorist group has made no comments on the blast.

The Aleppo Citadel contains remains of historic mosques, a palace, and bath buildings.

The blast comes days after the militants launched an offensive against government-held districts in Aleppo, firing rockets into at least seven neighborhoods controlled by Syrian forces.

Back in March, militants, including members of the al-Qaeda-linked al-Nusra Front, attacked a base in Aleppo used by the Syrian army.

ISIL also seized the ancient Syrian city of Palmyra back in May. The historic city has witnessed fighting since.

The violence fueled by Takfiri terrorist groups in Syria has reportedly left over 230,000 people dead and millions displaced since March 2011.